As for solutions I probed around for ideas a bit back with minor success, never got around to writing any proof of concept code to test any of it though.
Most of the ideas would probably end up being either too computationally expensive compared to what was saved or end up too difficult to implement but I am still interested in what rklaehn brought up with utilizing bloom filters, this would potentially cull a huge number of wantlist requests to peers that dont have what you are looking for.